Methods for processing converted wave seismic data

Abstract

The present invention provides methods for processing converted wave seismic data which includes, fractional point gathering of the data in a manner consistent with a selected velocity model, dynamic correction of the data using parameters measured from the data to account for the asymmetric travel path of the converted wave rays and stacking the dynamically corrected data. Methods are also provided for updating the velocity model.

What is claimed is: 
     
       1. A method for improved resolution of reflection seismic data containing at least converted wave information, comprising: providing seismic field records employing known multiple source and receiver locations having a known acquisition geometry,   selecting an appropriate layered earth model representative of the area of interest having known parameters,   ray tracing through said model to determine reflection points, two-way travel times, and values representative of signal amplitudes for converted waves, for a plurality of offsets,   determining for said plurality of offsets the variance between each of a plurality of fixed fractional sort points and said determined reflection points projected to the surface for at least one depth range of interest wherein said variance includes amplitude versus offset effects from said determined signal amplitudes and includes the effects of the geometry employed to generate the seismic field records,   determining a fixed fractional sort point that minimizes said variance for all the seismic field records in the area of interest, and   gathering the field records using said determined fixed fractional sort point.   
     
     
       2. The method described in claim 1, further comprising: (a) determining a second model representative of said area of interest having selected values for some parameters and known values for other parameters,   (b) determining for said second model two-way travel times for offset values representative of said seismic records,   (c) correcting said records for each offset value and various travel times by an amount dependent upon said various travel times and their corresponding zero offset times,   (d) determining the quality of correction of said records for said parameters,   (e) repeating the above steps a-d for different selected values for said some parameters,   (f) choosing values of said parameters that maximize the quality of correction, and   (g) correcting said records for each offset value and various travel times by an amount functionally related to said chosen values.   
     
     
       3. The method as described in claim 2, further comprising: determining from said parameters converted wave effective velocity and zero-offset time at the top and bottom of at least one layer of said layered earth model,   determining compressional and shear wave interval velocities and layer thickness for said at least one layer from said effective velocities and zero-offset times, and   changing said layered earth model to incorporate said determined thickness and velocities.   
     
     
       4. The method as described in claim 3, further comprising: redetermining said reflection points, two-way travel times, and values representative of signal amplitudes for said plurality of offsets, redetermining for said plurality of offsets said variance,   redetermining a fixed fractional sort point, and   regathering said field records with said redetermined fixed fractional sort point.     
     
     
       5. The method as described in claim 4, further comprising: redetermining said second model and two-way travel times for said offset values,   recorrecting said records,   redetermining said function,   reselecting said values, and   recorrecting said records.   
     
     
       6. The method as described in claim 2, further comprising: stacking said corrected records.   
     
     
       7. The method described in claim 2, further comprising: correcting said records for static variations.   
     
     
       8. The method described in claim 7, further comprising: stacking said records corrected for static variations.   
     
     
       9. A method for gathering converted wave seismic data, comprising: generating seismic field records employing multiple source and receiver locations,   selecting an appropriate earth model representative of the area of interest having known parameters,   selecting a plurality of fixed fractional sort points based upon at least said model for said area of interest,   determining reflection points, two-way travel times and values representative of signal amplitudes for said model,   determining the variance between each of said reflection points projected to the surface and of each said sort points for at least one depth range of interest wherein said variance includes amplitude versus offset effects from said determined signal amplitudes and considering the geometry employed to acquire the seismic field records,   determining that said variance is less than some preselected variance for the area of interest for all the seismic field records in the area of interest, and   gathering the field data using the selected fixed fractional surface sort point.   
     
     
       10. A method for dynamically correcting gathered converted wave seismic data, comprising: a. providing selected values of normal incidence travel time, T OPS , effective velocity, V ePS , P-wave apparent anisotropy, A P  *, and S-wave apparent anisotropy, A S  *, as well as the known functions, PP effective velocity, V eP  (T OPP ), and section dip, D PP  (T OPP ) (or D PS  (T OPS )) to obtain a single layer effective model having parameters: normal depth to reflector Z, P-wave vertical velocity V P , S-wave vertical velocity V S , P-wave apparent anisotropy A P  *, S-wave apparent anisotropy A S  *, and reflector dip angle, α;   b. determining the corresponding travel times T(X) for each offset represented in the seismic data from ray-tracing through said model;   c. selecting a sample of data from a seismic trace of said seismic data with offset X for each value of T(X) generated in step (b);   d. shifting said sample of data by an amount corresponding to the difference between T(X) and T OPS  ;   e. repeating steps b-d for each offset represented in the seismic traces of said seismic data;   f. detecting the signal in such shifted by summing or correlating the samples over all offsets;   g. repeating steps a-f for different iterated values of velocity V ePS , and for different values of T OPS  ;   h. selecting values of V ePS  for each T OPS  which cause the shifted data to have maximum signal amplitude; and   i. correcting the data with said selected values of V ePS .   
     
     
       11. A method for processing converted wave data to provide improved resolution, comprising: gathering said data with a fractional sort point to minimize any lateral smear that includes amplitude versus offset effects for at least one time interval of said data,   dynamically correcting said gathered data with a shear and compressional velocity to minimize any vertical smear for said at least one time interval,   correcting said data for static variations, and   stacking said data corrected for static variations.   
     
     
       12. A method for dynamically correcting gathered converted wave seismic data, comprising: (a) determining a model having selected values for some parameters and known values for other parameters,   (b) ray-tracing through said model to determine two-way travel times for offset values representative of seismic data from an area of interest,   (c) adjusting said data for each offset value and various travel times by an amount dependent upon said various travel times and their corresponding zero offset times,   (d) determining the quality of correction of said data for said parameters,   (e) repeating the above steps a-d for different selected values of said some parameters,   (f) choosing values of said selected values of said some parameters that maximize the quality of correction, and   (g) adjusting said data for each offset value and various travel times by an amount dependent upon said chosen values of said selected values of said some parameters and known values for said other parameters.   
     
     
       13. A method for determining at least one shear interval velocity from converted wave seismic data, comprising: determining a compressional velocity model for an area of interest;   determining converted wave effective velocities and zero-offset travel times at the top and bottom of at least one layer from converted wave dynamic correction velocity functions;   determining thickness for each said layer from an initial provided estimate, from said compressional velocity model and from said converted wave effective velocities and zero-offset travel times for said at least one layer; and   determining a shear velocity for each said layer from said thickness, said compressional velocity model, and said converted wave effective velocities.   
     
     
       14. A method for processing converted wave seismic data, comprising: generating seismic field records employing known multiple source and receiver locations,   selecting trial fixed fractional sort points for portions of said field records,   estimating the lateral smear of converted wave reflection points for each of said trial sort points, and   gathering said field record using a trial fixed fractional sort point that minimizes said lateral smear, wherein said lateral smear includes amplitude versus offset effects.   
     
     
       15. A method as described in claim 14, further, comprising: determining the trial surface sort point that minimizes said lateral smear, and wherein   said gathering said field records step uses said determined surface sort point.   
     
     
       16. A method as described in claim 15, wherein said estimating the lateral smear step, comprises: determining reflection points for said portions of said field records, and   determining the variance between surface projections of said determined reflection point and each of said trial fixed fractional surface sort points.   
     
     
       17. The method as described in claim 16, wherein said determining reflection points step, comprises: determining a model for the area covered by said portions of said field records, and   ray tracing through said model to determine said reflection points.   
     
     
       18. A method for dynamically correcting gathered converted wave seismic data, comprising: generating seismic field records employing known multiple source and receiver locations,   selecting a plurality of trial velocities and zero offset travel times,   determining a model representative of portions of said field records,   ray tracing through said model using said plurality of velocities and travel times to determine two way travel times for various offset values,   dynamically correcting said records for said plurality of velocities and travel times using said determined two way travel times from said raytracing step,   determining for each travel time the velocity of said plurality of velocities that provides the optimal dynamic correction, and   dynamically correcting said records with said determined velocity.   
     
     
       19. A method for velocity analysis of gathered converted wave seismic data, comprising: generating seismic field records employing known multiple source and receiver locations,   selecting a plurality of trial velocities and zero offset travel times,   determining a model representative of portions of said field records,   ray tracing through said model using said plurality of velocities and travel times to determine two way travel times for various offset values,   dynamically correcting said records for said plurality of velocities and travel times using said determined two way travel times from said raytracing step, and   determining for each travel time the velocity of said plurality of velocities that provides the optimal dynamic correction.   
     
     
       20. A method for velocity analysis of gathered converted wave seismic data, comprising: (a) determining a model having selected values for some parameters and known values for other parameters,   (b) ray-tracing through said model to determine two-way travel times for offset values representative of seismic data from an area of interest,   (c) adjusting said data for each offset value and various travel times by an amount dependent upon said various travel times and their corresponding zero offset times,   (d) determining the quality of correction of said data for said parameters,   (e) repeating the above steps a-d for different selected values of said some parameters,   (f) choosing values of said selected values of said some parameters that maximize the quality of correction, and   (g) determining said velocities from said chosen values.   
     
     
       21. A method for velocity analysis of gathered converted wave seismic data, comprising: a. providing selected values of normal incidence travel time, T OPS , effective velocity, V ePS , P-wave apparent anisotropy, A P  *, and S-wave apparent anisotropy, A S  *, as well as the known functions, PP effective velocity, V eP  (T OPP ), and section dip, D PP  (T OPP ) (or D PS  (T OPS )), to obtain a single layer effective model having parameters: normal depth to reflector Z, P-wave vertical velocity V P , S-wave vertical velocity V S , P-wave apparent anisotropy A P  *, S-wave apparent anisotropy A S  *, and reflector dip angle, α;   b. determining the corresponding travel times T(X) for each offset represented in the seismic data from ray-tracing through said model;   c. selecting a sample of data from a seismic trace of said seismic data with offset X for each value of T(X) generated in step (b);   d. shifting said sample of data by an amount corresponding to the difference between T(X) and T OPS  ;   e. repeating steps b-d for each offset represented in the seismic traces of said seismic data;   f. detecting the signal in such shifted samples by summing or correlating the samples over all offsets;   g. repeating steps a-f for different iterated values of velocity V ePS , and for different values of T OPS  ; and   h. selecting values of V ePS  for each T OPS  which cause the shifted data to have maximum signal amplitude.   
     
     
       22. A method for determining at least one shear interval velocity from converted wave seismic data, comprising: determining a compressional velocity model for an area of interest;   determining converted wave effective velocities and zero-offset travel times at the top and bottom of at least one layer from a converted wave dynamic correction velocity function for the area of interest;   providing an initial estimate of said at least one layer thickness and of shear and compressional anisotropy factors;   determining a revised thickness for each said layer from said initial estimate, said compressional velocity model, said converted wave effective velocities, said zero-offset travel times, and shear and compressional anisotropy factors for said at least one layer;   repeating the determination of said revised layer thickness with the initial thickness for each iteration being the revised thickness resulting from the previous iteration; and   determining a shear velocity corresponding to the revised layer thickness last determined.